The present invention relates to markers for surveillance or control and to systems employing the same. Such markers and systems are now used extensively to prevent shoplifting and similar unauthorized removal of articles from a controlled area.
The known marker systems tend to fall into one or the other of two general categories. One category makes use of radio frequency signals, usually in the microwave range, and is typified by the system disclosed in U.S. Pat. No. 4,063,229, issued on Dec. 13, 1977, to John Welsh and Richard N. Vaughan for "Article Surveillance." There is described therein a system wherein sensor-emitter labels or tags containing a semiconductor diode or the like are applied to articles for the purpose of surveillance. Said patent also describes the construction of special tags containing layers of ferrite material that can be magnetized or demagnetized by a suitable magnetic field for altering the operating characteristic of the tag and thereby deactivating the same.
An improved RF system, one that combines a microwave carrier signal with a low frequency signal that is used to establish an electrostatic field, is described in U.S. Pat. No. 3,895,368, issued July 15, 1975, to Lloyd L. Gordon and Robert D. Williamson for "Surveillance System and Method Utilizing Both Electrostatic and Electromagnetic Fields."
The second category makes use of electromagnetic fields inductively coupled to the marker which contains a high permeability low coercivity material having the ability of producing detectable harmonic frequencies when immersed in an alternating magnetic field of sufficient intensity. A refined system falling into this second category is described in copending United States patent application Ser. No. 193,038, filed on Oct. 2, 1980 by Jon N. Weaver for "Magnetic Surveillance System With Odd-Even Harmonic and Phase Discrimination," now U.S. Pat. No. 4,309,697, issued Jan. 5, 1982. Said Weaver patent also contains a good revue of the prior work in the area of this category. The above-mentioned patents are assigned to the same assignee as the present invention and application.
One of the patents mentioned in said Weaver patent is that of Bakeman, Jr. et al., U.S. Pat. No. 3,983,552, issued Sept. 28, 1976. There is disclosed therein a pilferage deterrent marker of laminated construction containing an easily magnetized layer of Permalloy and a control layer of difficult to magnetize vicalloy of remendur. Such marker, when the control layer is magnetized, is detected by a circuit responding to the amplitude and phase of the received second harmonic signal. When the control layer is demagnetized the marker is permitted to pass undetected.
A reverse deactivation arrangement wherein the marker is detected only when demagnetized is described in U.S. Pat. No. 3,820,104, issued on June 25, 1974, to Edward R. Fearon for "Methods and System for Detecting an Object Within A Magnetic Field Interrogation Zone." The abstract in said patent describes the marker as including a first elongated ferromagnetic element for being secured to the object. The first ferromagnetic element has a relatively low coercivity and is operable to generate a detectable signal containing harmonics of the fundamental frequency when placed in the interrogation zone. The marker further includes a second ferromagnetic element disposed adjacent to the first element and having a coercivity greater than the first element. Deactivation structure is provided to selectively magnetize the second ferromagnetic element to impose a plurality of pairs of alternate magnetic poles on the first element in order to deactivate the marker. The deactivated marker does not generate a detectable signal containing the desired harmonics when the object passes through the interrogation zone.
An analysis of the development history of both categories of marker systems reveals a continuing effort to improve sensitivity while reducing false alarms due to triggering of the detection equipment by components other than the markers for which the system was designed. Early RF systems were triggered by transistor radios, an obvious potential problem because of the diodes and other non-linear devices normally incorporated therein, and by the less obvious such as a baby carriage with a rusty oxidic junction therein. Comparable problems have also confronted the magnetic systems.